Harvesting ambient RF power is attractive as a means to operate microelectronics without wires, batteries, or even a dedicated RFID reader. However, previous ambient RF harvesters have been narrowband, making mobile sensing scenarios infeasible: for example, an RF harvester tuned to work in one city will not generally work in another, as the spectral environments tend to differ.
Ambient radio harvesting is a promising approach to powering battery-free sensing, computing, and communication devices. The vanishingly small amount of power now required by modern microelectronics, along with low power communication techniques, make ambient radio harvesting an increasingly viable power supply option. Compared to solar power, ambient RF has the advantage of being available at night, and is attractive from an industrial design perspective: the antennas already designed in to mobile devices for communication purposes can potentially become a power source, without requiring any changes to the form factor or appearance of the device.
However, there still exist challenges to widespread adoption of ambient RF harvesting as a power source. Conventional RF harvesting methods are only capable of extracting power from a narrow spectral band. The supply will cease to provide power when its particular source band is not available, either due to geographical fluctuations in spectral occupation, occlusion and shielding (e.g., from the walls of a building), or simply from multipath fading of the ambient signal. When starved of its energy source the device must cease to operate, limiting the application space mostly to a small geographical area with line-of-sight to an ambient radio source of interest.
Wideband harvesting may capture energy across a large swath of spectrum, but typically results in very low efficiency at any particular source frequency as the quality of the impedance match between the antenna and single rectifier reduces as the bandwidth increases. Tunable harvesting may allow a system to dynamically select a band of interest based on spectral availability, and therefore promises to be able to provide efficient rectification of signals from a single source, regardless of the frequency of that source. However, tunable harvesters in battery free systems may have difficulty bootstrapping to allow the system to coldstart, and also ignore energy outside the band to which they are tuned.
Existing multiband harvesting generally makes use of multiple antennas, each tuned to a band of interest, and each feeding an independent rectifier through a tuned matching circuit.